The present invention relates to a formulation of polyvinyl chloride (PVC) compounds, particularly to high mechanical resistance compounds specially with regard to abrasion resistance and that do not contain stabilizers based on heavy metals.
Hereinafter the polyvinyl chloride will be referred to as PVC. In the art, it is known as a product used extensively in insulation and covers for electric conductor cables because of its low price and its availability as well as because of its dielectric and mechanical properties and its chemical and environmental resistance.
For this kind of formulations, PVC is always used with the addition of plasticizers to remove its natural rigidity and to supply the wished flexibility. Other additive agents, such as thermic stabilizers, lubricants, pigments, charges, impact modifiers and flame retarders are included in the formulation to obtain a PVC with the wished properties.
PVC, on its own, is thermally unstable, because it decomposes at a temperature close to 150.degree. C. releasing HCI and producing unsaturated sites in the polymer thereby causing chain reticulation and rupture, resulting in the degradation of the polymer properties. While the PVC is being decomposed the resin changes color and becomes rigid and fragile.
In order to improve thermal stability, stabilizers for PVC compounds are applied. The most commonly used stabilizers are generally metal salts and organic or inorganic phenols, organometallics, epoxy compounds and phosphites. In the case of compounds designed for the insulation of automotive primary cables, PACKARD ELECTRIC ES M 2397 specification states that the compounds used for the manufacture of thin wall cables according to specification ES M 3089 should be lead free.
The automotive industry is being affected by the following factors: the legislation regarding the environment that compels the minimization of the environmental impact of the present vehicles both with regard to the emission they produce and to their construction and the wish to increase the efficiency, safety, luxury and comfort offered by the present vehicles.
These requirements have forced the vehicle manufacturers to modify the materials employed in the fabrication of the vehicles as well as their performance in use.
The first modification was the elimination of the noxious materials found in the polymeric compounds, among them PVC, which contained lead because of economic and electrical advantages.
In order to reduce the environmental Impact there have been improvements in the internal combustion engines to optimize fuel consumption. On the other hand, vehicles of smaller dimensions and weights have been designed and spaces in the various compartments have been reduced, leaving less space for the devices and their connection harnesses, that are ever more numerous to improve both safety and luxury. For these reasons, vehicle manufacturers have modified the cable designs, reducing the insulation wall thickness to diminish weight and diameter in order to increase the number of circuits within the same space. Said walls thickness reduction should not affect their performance.